1. Field of the Invention
The invention relates generally to the drilling of a lateral hole from a main well.
2. Background Art
Lateral hole drilling has become a new drilling method to construct a well. With the lateral hole drilling allows to access an extra zone of an underground reservoir, e.g. an hydrocarbon reservoir, or an aquifer. The lateral hole drilling method is proven to be useful in the case of high hydrocarbon viscosity, low permeability formation, highly layered reservoir etc. The lateral hole drilling method also enables to reach a reservoir when drilling slots are limited, like for example with an off-shore platform.
A drilling rig is commonly used to drill the lateral hole departing from a main well. A rotating torque is generated at surface and is transmitted to a drill string downhole. The rotating torque may also be generated downhole by an hydraulic converter while a pump is used at surface. An axial force to be applied on a drill bit at an end of the drill string may be generated by the weight of the drill string along a vertical or diagonal portion of the main well.
A coiled tubing may also be employed for drilling the lateral hole. An injection head pushes a coiled tubing into the main well. Several tools, typically a drill collar, an orienting tool, a steerable motor and a drill bit, may be located at an end of the coiled tubing. A rotating torque and an axial force are applied on the drill bit. The rotating torque is generated by an hydraulic converter of the steerable motor while a pump is used at surface. The axial force may be generated by the weight of the tools, or even of the coiled tubing. The axial force may also be generated at surface by the injection head.
Several recent systems for drilling small lateral holes generate the rotating torque downhole with an electrical motor. In most cases, the drilling of the lateral hole is performed in two steps. During a first step, a short radius curved hole is drilled using a first drilling system. When a desired direction is reached, the first drilling system is removed out of the lateral hole and a second drilling system drills the lateral hole substantially following the determined direction.
The first drilling system may be a steerable motor that is bent so as to allow to drill following a curve.
Steerable Motor
FIG. 1 illustrates a schematic of a steerable motor according to prior art. The steerable motor 101 comprises a drill pipe 105, a transmission shaft 103 to which a drill bit 107 is connected. The drill pipe 105 is bent so as to allow to drill a curved hole. During the drilling, the steerable motor 101 is forced against a bottom wall of the drilled hole: a command radius of the curved hole is determined by relative positions of three contact points 102.
In case of a soft formation, it may happen that the steerable motor 101 drills a bore having a relatively large section. A resulting curved hole may hence have an effective radius that is higher than the command radius. In order to control the effective radius, the contact points 102 may be provided at locations corresponding to a relatively small command radius. The steerable motor 101 may be employed with either an angled mode or a straight mode.
In the angled mode, an hydraulic converter 104, e.g. a progressive cavity motor, located in the steerable motor rotates the transmission shaft 103 using a circulation of a drilling fluid (not represented). The drilling bit 107 is hence rotated. The drill pipe 105 remains at a same azimuthal position and transmits an axial force. The lower part of the transmission shaft 103 is supported by bearings 106 to transmit the axial force from the drill pipe 105 to the drill bit 107. As a result, the resulting curved hole is bent with an effective radius greater or equal to the command radius.
If the effective radius is smaller than a desired radius, the steerable motor 101 may be used in a straight mode, i.e., the drill pipe 105 itself is rotated. The bent angle fails to point in a preferred direction, and a large hole having a substantially straight direction is drilled. When combined to the angled mode, the straight mode allows to control the effective radius of the curved hole.
Control of a Direction of Drilling
During a drilling, a bottom hole assembly, such as the steerable motor, may comprise stabilizers. The stabilizers allow to position the drill pipe in the hole. The stabilizers also allow to drill in an upward direction, or in a downward direction.
FIG. 2 illustrates a stabilizer from prior art. The stabilizer 202 comprises blades that surrounds a drill string 201 and leans on an internal wall 204 of a drilled hole. Hence the stabilizer 202 maintains a center of the drill string 201 substantially in a center of a section of the drilled hole. The weight of the drill string may cause a deformation of the drill string. The drill string 201 hence allows to drill following a direction that is determined by relative longitudinal positions of the stabilizers and by the weight of the drill string 201.
FIG. 3A illustrates a straight configuration of a bottom hole assembly for drilling a lateral hole according to prior art. A drill bit 303 is located at an end of a drill string 301 of a bottom hole assembly. Three stabilizers (302a, 302b, 302c) surround the drill string 301 at different locations. The stabilizers (302a, 302b, 302c) maintain a center of the drill bit 303 in a center of a section of a drilled hole 304 so as to insure a relatively straight drilling.
FIG. 3B illustrates a drop configuration of a bottom hole assembly for drilling a lateral hole according to prior art. A first stabilizer 302a and a second stabilizer 302b surround a drill string 301. As the first stabilizer 302a and the second stabilizer 302b are located at a relatively high distance from a drill bit 303 at an end of the drill string 301, the drill string 301 flexes under its own weight, thus causing the drill bit 303 to drill a hole 304 following a downward direction.
FIG. 3C illustrates a build configuration of a bottom hole assembly for drilling a lateral hole according to prior art. A first stabilizer 302a and a second stabilizer 302c surround a drill string 301. The first stabilizer 302a and the second stabilizer 302c are located at a relatively long distance from each other, and the second stabilizer 302c is relatively close to a drill bit 303 at an end of the drill string 301. A weight of a portion of the drill string 301 between the stabilizers (302a, 302c) causes the drill string 301 to flex elastically downward between the stabilizers (302a, 302c). The drill bit 303 is hence pushed upward and drills in an upward direction.
When a change of direction is required, the drill string needs to be pulled out of the well so as to displace the stabilizers. In order to avoid the pulling out of the drill string, a variable diameter stabilizer may be set. The diameter of the variable diameter stabilizer may be changed from one position to the other. The changing of position involves a mechanical system: only one single different diameter of the variable diameter stabilizer may be set in a bottom hole assembly. The changing of position may be commanded from surface.
A setting of the variable diameter stabilizer is typically controlled by mechanical and flow events, e.g. an applying of an axial force, a removal of a rotating torque, an applying of a flow of a flow, a pressure drop due to the applying of the flow etc. A chronological order of the mechanical and flow events allows to set a proper stabilizer position. For example, the mechanical system typically comprises a key that may slide within an internal slot along a periphery of the bottom hole assembly. The key may slide between an upward position and a downward position depending on the chronological order of the mechanical and flow events. When the key is in the upward position, a transmission system allows a blade of the variable diameter stabilizer to be retracted. When the key is in the downward position, the transmission system pushes the blade against a wall of the drilled hole. The transmission system may be a shaft indirectly connected to the blade, or an inside tubing that is cone-shaped.
It is hence possible to decide from the surface if the drilling is performed following a straight direction or an other direction. The other direction may be an upward direction, or a downward direction, depending on a relative longitudinal position of the variable diameter stabilizer.
A bottom hole assembly with a variable diameter stabilizer may comprise three stabilizers as represented in FIG. 3A, wherein one of the three stabilizers is the variable diameter stabilizer. The variable diameter stabilizer may be the closest from the drill bit stabilizer. In this case, a retracting of the diameter of the variable diameter stabilizer provides a configuration that is similar to the one represented in FIG. 3B. It is hence possible to drill following a straight direction or a downward direction, depending on a diameter of the variable diameter stabilizer.
Similarly, the diameter stabilizer may be located between the other stabilizers. In this case, a retracting of the diameter of the variable diameter stabilizer provides a configuration that is similar to the one represented in FIG. 3C. It is hence possible to drill following a straight direction or an upward direction, depending on a diameter of the variable diameter stabilizer.
Monitoring of the Direction of Drilling
Controlling a direction of a drilling of a lateral hole also requires to monitor a drilling direction of a drill bit. Such a monitoring is usually performed by providing a Measurement While Drilling (MWD) tool on a bottom hole assembly. The MWD tool may comprise an accelerometer system and a magnetometer system. The accelerometer system comprises at least one accelerometer. The accelerometer allows a measurement of an inclination of a drill pipe versus the Earth gravity vector. The magnetometer system comprises at least one magnetometer allowing a measurement of an azimuth of the drill pipe versus the Earth magnetic field.
The accelerometer system may comprise three accelerometers allowing to measure three distinct inclinations versus the Earth gravity vector, so as to provide a three dimensions measurement of a position of the drill pipe.
The magnetometer system may comprise three magnetometers allowing to measure three distinct azimuths versus the Earth magnetic field. The MWD tool may also comprise both the three accelerometers and the three magnetometers.
The MWD tool typically communicates with the surface using acoustic telemetry. The MWD tool is typically located at a relatively high distance from the drill bit, e.g. 25 meters. As a consequence of this distance, the MWD provides measurements having a relatively low accuracy, since a curvature of the lateral hole below the MWD is not known.
Very Short Radius Drilling
In a case of a very short radius drilling, it is possible to use a motor that is blocked within a main well and a flexible shaft that may transmit a rotating torque and an axial force to a drill bit. The flexible shaft is bent substantially perpendicularly at an elbow between the main well and a drilled lateral hole. A guide system is provided within the main well so as to allow the transmitting of the rotating torque and the axial force at the elbow.
The guide system may be lubricated so as to diminish contact stresses between the flexible shaft and the whipstock.
The guide system is typically a whipstock.
International application WO99/29997 describes a system in which bushings are used within an elbow for causing a flexible shaft to flex and turn while permitting rotation and axial movement therethrough.
Flow and Cuttings Management
Drilling a hole creates cuttings that need to be processed. This can for example de done as described in the following. A pump at surface injects a drilling fluid, e.g. a drilling mud, through a hollow drilling tool. The drilling fluid reaches a drill bit of the drilling tool and is evacuated through an annulus between the drilling tool and the drilled hole. The drilling fluid is viscous enough to carry the cuttings that are created at the drill bit up to the surface. A shale shaker located at the surface allows to separate the cuttings from the drilling fluid.